The invention relates to a method of and device for forming a tripode star for a tripode joint, which tripode star comprises an annular hub member and three arms which adjoin the hub member, extend radially relative to the longitudinal axis of the hub member and which each form faces of spherical portions positioned circumferentially relative to the arm axes of the arms. Tripode stars of said type are used in tripode joints wherein roller carriers for the rollers are arranged on the tripode arms, which roller carriers are each provided with axial bores engaged by the tripode arms in a substantially play-free way. In this way, the roller carriers are axially displaceable and swingingly movable relative to the tripode arm. In this way, it is ensured that, when an outer joint part and an inner joint part of a tripode joint rotate with one another in an angular position relative to one another, the tripode rollers supported on the roller carriers can be guided, so as to carry out a pure rolling contact movement in the tracks of the outer joint part, with the axes of the tripode rollers always being parallel relative to themselves.
Tripode stars/inner joint parts of the above-mentioned type have so far been produced by using a cylindrical blank for forming the hub member and the three arms by forward extrusion by means of a tool divided in the plane containing the arm axes, as a result of which there occurs a burr extending in the central plane of the tripode star. During a subsequent production stage, which is not essential in this connection, first, the hub member is provided with a bore. This operation is followed by machining processes removing the burr which emerged during the deformation of the blank at least in the region of the faces of the spherical portions at the arms, which faces have to comprise a good surface quality as functional faces, which cooperate with the axial bores of the roller carriers. Said machining processes can take place in the form of hard-turning or grinding carried out at the already hardened tripode star.
It is the object of the present invention to simplify the forming method used for the tripode stars of tripode joints of said type.
The objective is achieved in that, first, as before, in a first production stage, the hub member and the three arms are pre-formed from a blank by means of a tool divided in the plane containing the arm axes and that, in a second production stage, pairs of burr-free faces of spherical portions positioned opposite one another in the circumferential direction relative to the hub member are formed on to the pre-formed arms by means of a tool divided in planes extending through the longitudinal axis and through the arm axes. During the second production stage, the burr which emerged during the first production stage is removed, and there are produced finished functional faces which do not need any subsequent machining and whose surface roughness is clearly reduced. It is appreciated that, during the first production stage, the faces of the spherical portions at the arms have to be pre-formed as far as possible, but still comprising the material allowance required for the second production stage.
It is possible, for the centers of the provisional faces of the spherical portions, after the first production stage, still to be positioned at a certain distance from the longitudinal axis and to assume their final position on a so-called pitch circle diameter only during the second production stage.
According to a particularly preferable embodiment, it is proposed that, in the course of the first production stage, the arms are formed so as to comprise flattened portions arranged opposite one another in the direction of the longitudinal axis of the hub member. In this way it becomes possible, during the second production stage, for the arms to grow in this region, without their design-related functions being adversely affected, i.e. the remaining flattened portions are still positioned inside the spherical contour complementing faces of the spherical portions and thus within the bore diameter of the roller carrier. Analogously, the same applies to the arm heads radially growing into an outer free space in the tools for the purpose of carrying out the second production stage.
According to a first embodiment of the inventive method, the tripode star, during the second production stage, can be held radially floatingly relative to the second tool in that clamping tongs for example hold the hub member in an accurate axial alignment only.
According to a second embodiment of the inventive method, the tripode star, during the second production stage, can be held so as to be firmly supported relative to the second tool, for example in a third tool which is divided in a plane containing the arm axes.
The forming process carried out on the arms in the second production stage can take place in three individual operations or in one single forming operation.
In any case, an inventive tool for carrying out the second production stage is characterised by at least one set of tools including of two symmetric mold halves for forming a tripode arm, which tool set is divided in a plane extending through the longitudinal axis and through an arm axis. These characteristics apply regardless of whether the second production stage is carried out for each arm individually or for all three arms simultaneously. In a preferred embodiment, the at least one tool set is open at the arm end faces positioned opposite one another in the direction of the longitudinal axis.
In one embodiment of the tool for carrying out the second production stage at one tripode arm only, the mold halves of the one single tool set are designed so as to be linearly guided and movable in opposite directions perpendicularly relative to the dividing plane.
A further embodiment of the tool for simultaneously carrying out the second production stage on all three arms is characterised in that the tool comprises three circumferentially distributed tool sets with mold halves with outer tool faces positioned approximately radially relative to the longitudinal axes and with three wedge elements which are positioned between the tool sets, which can be moved radially inwardly and whose wedge faces cooperate with the outer tool faces of adjoining tool sets. All mold halves can be designed so as to be movable independently of one another and held in a cylindrical ring, or there can be provided three first mold halves which are independent of one another and are held in a cylindrical ring, whereas three second complementary mold halves are firmly connected to one another by a disc, or three first mold halves can be firmly connected to one another by a first disc, with three second complementary mold halves being firmly connected to one another by a second disc.
A further embodiment of the tool for carrying out this production stage is characterised in that the tool comprises three circumferentially distributed tool sets with mold halves with outer tool faces which are arranged in pairs, which extend parallel to the arm axes and which are positioned wedge-like relative to one another and with three wedge elements which can be moved axially in the longitudinal direction, which are positioned between the tool sets and which are supported in a cylindrical ring and which, by means of their wedge faces, cooperate with the outer tool faces of adjoining tool sets.
One tool, by means of which the arms can be machined only individually during the second production stage, but which permits the complete tripode star to be securely supported during this forming operation is characterised in that the mold halves are provided in the form of a pair of punches which are guided in transverse bores of a holding tool which receives the complete tripode star and which is divided in the plane containing the arm axes.